The nitrate and phosphate did not kill your fish and snails, so you should keep looking for a cause. Are you testing nitrite? It is toxic and could be spiking now.
As far as I’m aware this is not true. There are some polymers in development that may someday do this, but it won’t be Poly Filter. Dialysis is more analogous to RO with selective diffusion of small waste molecules and electrolytes across a dialysis filter.(they're used in kidney dialysis from what I understand),
Wait! Do not adjust salinity too quickly. Rapid change can be really bad. Although down is better than up.Outing myself...tested all parameters today. Salinity was 50! somehow the water changes I've been doing jacked things up. Fortunately I had some RO and did a quick change. down to 40 and tomorrow will right the ship with another change. I feel terrible. The clowns have been fine, but the blenny was super unhappy and checked out. The blue tip NEM is holding its own right now. Pretty big lesson learned...need another refractometer for sure!